Heating, ventilation, and/or air conditioning (HVAC) systems are commonly used to control the comfort level within a building or other structure. There are various types of HVAC systems that are in use today. Some HVAC systems affect conditions in multiple rooms of a building with a centralized source of heated or cooled air (e.g., central furnace, air conditioner and/or air handler) and a network of ducts, dampers, etc., to manage airflow throughout the building. Other HVAC systems are more limited in extent, such as self-contained window air conditioner. Some HVAC systems, such as mini-split HVAC systems, include an indoor unit that circulates cooled and/or heated air inside a room of the building without a network of ducts, and an outdoor unit connected to the indoor unit via lines carrying a working fluid that transfers heat between the indoor unit and the outdoor unit.
Many HVAC systems include a controller that activates and deactivates one or more HVAC units or components of the HVAC system to affect and control one or more environmental conditions within the building. These environmental conditions can include, but are not limited to, temperature, humidity, and/or ventilation. In many cases, such an HVAC controller may include, or have access to, one or more sensors, and may use parameters provided by the one or more sensors to control the one or more HVAC components to achieve desired programmed or set environmental conditions.
An HVAC controller may be equipped with a user interface that allows a user to monitor and adjust the environmental conditions at one or more locations within the building. With more modern designs, the interface typically includes a display panel, such as a liquid crystal display panel, inset within a housing that contains a microprocessor as well as other components of the HVAC controller. In some designs, the user interface may permit the user to program the controller to control to a certain schedule that is determined by the user. For example, the interface may include a routine that permits the user to set the desired temperature at one or more times during a particular day and/or group of days. In many cases, such a programmable schedule may help reduce energy consumption of the HVAC system by changing the setpoint to an energy saving set back temperature during certain times, such as when the building or space is expected to be unoccupied or when the occupants are expected to be sleeping.
Some HVAC control systems provide an auto changeover mode, in which an HVAC controller may automatically select between the heating and cooling modes of the HVAC system. Some HVAC control systems, such as many mini-split HVAC systems, have an onboard controller that implements such an auto changeover mode using a single or common setpoint. In such systems, the onboard controller of the HVAC system typically commands the HVAC system to heat the air when the current sensed temperature is below the single setpoint, and cool the air when the current sensed temperature is above the single setpoint.
A problem may arise when a single setpoint auto changeover mode is employed in conjunction with a programmable schedule. To illustrate this, one may contemplate a scenario during a summer cooling season, where a cooling setpoint is changed from a setpoint of 72 degrees when an inside space is expected to be occupied (Home), to a setback energy savings temperature setpoint of 80 degrees when the inside space is expected to be unoccupied (Away). In an HVAC control system that has an auto changeover mode that uses a single or common setpoint, the auto changeover logic may command the HVAC system to activate the heat mode when the setpoint changes from 72 degrees to 80 degrees because the sensed inside temperature will be below the new set point temperature of 80 degrees. A similar situation may occur during the heating season. For example, one may contemplate a scenario during the heating season where a heating setpoint is changed from a setpoint of 72 degrees when an inside space is expected to be occupied (Home), to a setback energy savings temperature setpoint of 68 degrees when the inside space is expected to be unoccupied (Away). In an HVAC control system that has an auto changeover mode that uses a single or common setpoint, the auto changeover logic may command the HVAC system to activate the cool mode when the setpoint changes from 72 degrees to 68 degrees because the sensed inside temperature will be above the new set point temperature of 68 degrees. Activating the heating system during the cooling season and/or activating the cooling system during the heating season will tend to reduce the overall energy efficiency of the system.